Intake and/or exhaust gas sensors may be operated to provide indications of various exhaust gas constituents. For example, US 20120037134 describes detecting engine intake dilution using an intake gas oxygen sensor. In alternate approaches, engine dilution may be estimated by an exhaust gas oxygen sensor. The estimated engine dilution may be used to adjust various engine operating parameters, such as fueling and air-fuel ratio. As another example, U.S. Pat. No. 5,145,566 describes detecting water content in the exhaust gas using an exhaust gas oxygen sensor. In alternate approaches, water content in exhaust gas recirculated to the engine intake (EGR) may be estimated using an intake gas oxygen sensor. Water content estimated using an intake or exhaust gas oxygen sensor may be used to infer an ambient humidity during engine operation. Further still, the water content may be used to infer an alcohol content of a fuel burned in the engine.
However, the inventors have recognized that oxygen sensors (both intake and exhaust oxygen sensors) can have significant part-to-part variability. For example, without any compensation, the variability in oxygen measurement by the sensor can be in the range of 15%. This variability in the sensor output can lead to a substantial error in the measurement of fuel alcohol content and engine dilution. For example, based on the variability of the sensor, an alcohol transfer function (used to estimate fuel alcohol content based on the oxygen sensor output) may vary. If a known transfer function for a nominal sensor is used, the fuel alcohol content may be overestimated or underestimated. As such, to correctly measure the fuel alcohol content, the oxygen sensor output needs to be compensated for this part-to-part variability which is affected not only by the age of the sensor, but also ambient conditions (in particular, ambient humidity levels), as well as the presence of additional diluents (such as purge or crankcase ventilation vapors).
The above issues may be addressed and accuracy of fuel alcohol content estimation by an (intake or exhaust) oxygen sensor can be improved by a method that better compensates for sensor part-to-part variability. One example method comprises, during selected conditions, operating an oxygen sensor at a lower reference voltage where water molecules are not dissociated to generate a first output and at a higher reference voltage where water molecules are fully dissociated to generate a second output. The method further comprises learning a correction factor for the sensor based on the first and second output. The method may further comprise adjusting a parameter based on an alcohol content, the alcohol content of fuel combusted in the engine estimated based on each of the first output and the learned correction factor for the sensor. In this way, oxygen sensor reliability is improved.
In one example, during selected conditions, the oxygen sensor is operated to determine an oxygen sensor reading corrected for dry air conditions. For example, during conditions when purge and crankcase ventilation gases are not ingested in an engine intake manifold, the reference voltage of an intake oxygen sensor may be modulated. Alternatively, in embodiments where the oxygen sensor is an exhaust oxygen sensor, the selected conditions may include engine non-fueling conditions, such as a deceleration fuel shut-off (DFSO) event. Specifically, the reference voltage of the oxygen sensor may be raised from a first, lower voltage where the output (e.g., pumping current) is representative of an oxygen reading in humid conditions, to a second, higher voltage where the output (e.g., pumping current) is representative of an increase in oxygen due to the full dissociation of humid. A dry air pumping current may then be determined based on a ratio between the first output and the second output, the dry air pumping current indicative of an oxygen reading in dry air. The dry air oxygen reading (the ratio between the first and second output) is then used to determine an alcohol transfer function correction. The corrected transfer function and the humid air oxygen reading (first output) may then be used to estimate a fuel alcohol content. The estimated fuel alcohol content can then be used to estimate an engine operating parameter, such as a desired air-fuel ratio for combustion. As an example, the controller may adjust an air-fuel ratio correction based on the estimated fuel alcohol content.
In this way, the part-to-part variability of an intake or exhaust oxygen sensor may be better learned, including part-to-part variability due to sensor aging. By learning the variability, the need for a compensation resistor configured to compensate for the part-to-part variability is reduced, providing cost and component reduction benefits. By using a dry air oxygen estimate provided by the oxygen sensor to correct an alcohol transfer function, irregularities in fuel ethanol estimation may be reduced. Overall, reliability of the sensor output is increased. Further, accuracy of fuel alcohol estimated based on oxygen sensor output is also increased. Since the sensor output and fuel alcohol estimate are used to adjust various engine operating parameters, overall engine performance is improved.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.